The invention relates to a hydraulic machine with a piston and slider shoe unit, in which the piston and the slider shoe are joined to one another by way of a ball-and-socket joint forming a first contact surface, and the slider shoe lies via the intermediary of a second contact surface on a control surface, a friction-reducing layer being arranged on one contact surface.
A hydraulic machine of that kind can operate according to the axial piston principle or according to the radial piston principle. In both cases the movement of the piston is controlled by way of a control surface on which the slider shoe lies and over of which it is guided during movement of the rail. When the control surface is inclined, the angular position of the slider shoe with respect to the piston changes during operation, as is the case, for example, with an axial piston machine having an inclined wobble plate.
In a known hydraulic machine (DE-OS 21 18 712) various principles are known to fix the slider shoe to the piston by means of a ball-and-socket joint. For that purpose, the ball and the slider shoe are interlockingly engaged with one another by means of a joining element; measures are taken to ensure that the ball of the ball-and-socket joint is mounted in the slider shoe so that the required rotary movement between the slider shoe and the piston is possible. US 3 183 848 describes a pump operating according to the axial piston principle, in which the slider shoes are made of nylon and are secured to the ball of the ball-and-socket joint by means of a metal clip.
During operation of the machine, friction occurs between the slider shoe and the control surface and between the slider shoe and the piston in the ball-and-socket joint, through the movement of the respective parts relative to one another. So that wear and tear and friction loss do not become too great, the contact surfaces are therefore lubricated. The hydraulic fluid that is already present is used for lubrication here. As a consequence, however, the choice of hydraulic fluids is restricted to those liquids that have a satisfactory lubrication. These are essentially synthetic oils, which are being regarded with ever increasing disfavour in the expanding debate on environmental protection now in progress. Replacing these oils by other liquids is possible only to a limited extent, since, as already mentioned, lubrication is not ensured in all cases.
In a machine of the kind mentioned in the introduction (JP 2-125 979 A), it is known to provide a friction-reducing layer comprising a plastics material mixed with fibres between the slider shoe and the control surface.
Fixing a plastics material of that kind to the slider shoe is, however, relatively complicated. The surface to be provided with the layer needs to be roughened or grooved, and the friction-reducing layer is then supposed to be adhesively secured to that surface. Because the adhesive bond is stressed primarily by shearing forces, there is a risk that the bond will not hold for long and the friction-reducing layer will therefore become detached, which leads to damage to the machine. With the known machine there furthermore the danger that too much friction will develop in the ball-and-socket joint, which can ultimately lead to this joint seizing up or binding. This would also result in damage to the machine.